The present invention relates to an ignition timing control system for an internal combustion engine in which the output of the internal combustion engine is improved by controlling the ignition timing according to knockings.
A configuration of an ignition timing control system for controlling the ignition timing in accordance with knockings is shown in FIG. 1. The ignition timing control system shown in this diagram comprises a knocking sensor 1 for detecting a knocking, a filter 2 for selectively passing the knocking component of the output frequency produced from the knocking sensor 1, a knocking decision level generator circuit 3 for generating a knocking decision level, a comparator circuit 4 for comparing the knocking decision level 6 with the output of the filter 7 thereby to make a decision on the knocking and an ignition timing control circuit 9 for controlling the ignition timing in accordance with the output signal of the comparator circuit 4.
A conventional knocking decision level generator circuit 3, as shown in FIG. 2, comprises an amplifier circuit 12 for amplifying the output of the filter 2, a half-wave rectifier circuit 10 for half-wave rectifying the output of the amplifier circuit 12, and an integrator circuit 11 for integrating the output of the half-wave rectifier circuit 10. The DC output of the integrator circuit 11 is applied as a knocking decision level to the comparator circuit 4. This knocking decision level generator circuit 3 enables the setting of a reference level for the comparator circuit 4 corresponding to the noise components of the output of the filter 2, so that an output 8 substantially corresponding to the knock component among the outputs of the filter 2 is obtained from the comparator circuit 4.
In this conventional ignition timing control system, the knocking decision level generator circuit 3 includes the amplifier 12, the half-wave rectifier circuit 10 and the integrator circuit 11, each including an operational amplifier, resulting in an increased number of circuit component elements. This increases not only the product cost but also the number of points causing an offset, a temperature drift of the operational amplifiers or other errors, thereby making it difficult to generate an accurate knocking decision level.